It is known that a crack, such as incipient cracks that may occur in steam turbine blades, can grow to a critical length and may lead to a point of failure of the blade, which can result in costly damage of the steam turbine. One area that has been susceptible to such occurrences is the last row of blades in the low-pressure section of a multi-section steam turbine. The blades in this last row have a relatively large diameter and the operational environment is a relatively low-temperature (e.g., near room temperature) and foggy environment due to the presence of saturated steam and/or water droplets.
One technique that has attempted to address the foregoing issues relies on detecting vibration abnormalities in an operating steam turbine. In particular, this technique analyzes the vibration characteristics of blades of interest by means of a blade vibration monitor (BVM). This technique is able to indicate high stresses, which could lead to cracks and eventually to a catastrophic failure of the blade, however, this technique is unable to provide a direct and reliable indication as to whether a crack has formed in the blade and how such a crack may be developing. Additional limitations concomitant with this technique may be as follows: The BMV sensing is generally performed either during the commissioning of a new turbine or during the recommissioning after an outage of a field-deployed steam turbine. The interpretation of the BMV signals generally requires service personnel with substantial training. Thus, the use of this technique entails a labor-intensive service for a relatively short period of time rather than a continuous blade monitoring that can run automatically as the turbine operates. It is believed that prior to the present invention those skilled in the art have shied away from the use of infrared sensing techniques to address the foregoing issues in view of the substantial challenges involved in capturing data with enough contrast and resolution for a fast rotating steam turbine component in a relative low-temperature environment and in the presence of saturated steam.